In conventional fluidized catalytic cracking (FCC) processes, a relatively heavy hydrocarbon feed, e.g., a gas oil, is mixed with a hot regenerated cracking catalyst in the base of an elongated riser reactor and cracked to ligher hydrocarbons. The cracked products and spent catalyst are discharged from the riser and separated into a vapor phase and a catalyst phase. The catalyst passes through a stripper to remove entrained hydrocarbons from the catalyst, then catalyst is regenerated. The catalyst circulates between the reactor and the regenerator and transfers heat from the regenerator to the reactor, supplying heat for the endothermic cracking reaction.
Catalytic cracking processes are disclosed in U.S. Nos. 3,617,497, 3,894,923, 4,051,013, 4,309,279 and 4,368,114 (single risers) and U.S. Nos. 3,748,251, 3,849,291, 3,894,931, 3,894,933, 3,894,934, 3,894,935, 3,926,778, 3,928,172, 3,974,062 and 4,116,814 (multiple risers).
In U.S. No. 4,051,013, a naphtha feed and a gas oil feed are converted in the presence of an amorphous or zeolite cracking catalyst in a riser reactor to high octane gasoline.
Several FCC processes use a mixture of catalysts having different catalytic properties, e.g., U.S. No. 3,894,934 uses a mixture of a large pore zeolite cracking catalyst such as zeolite Y and a shape selective zeolite such as ZSM-5. The combined catalyst system (or mixture) produces a gasoline product of relatively high octane rating.
In U.S. No. 4,116,814, which is incorporated by reference, Zahner teaches use of two different kinds of catalyst, with catalyst separation in the fluidized regenerator. This approach will work, but when a low coke producing catalyst containing, e.g., ZSM-5, is used, it will spend a lot more time in the regenerator than is necessary and hence will experience more hydrothermal degradation than is necessary.
It would be beneficial if such a low coke-forming catalyst could be separated outside the regenerator.
The approach taken in U.S. No. 4,490,241, Chou, to keeping the ZSM-5 additive out of the regenerator is to make the additive very light, so that it can be collected in secondary cyclones downstream of the riser reactor. This patent is incorporated by reference. Use of very small, or light, particles of ZSM-5 additive which is recycled from secondary cyclones will work to keep the ZSM-5 out of the regenerator but will result in rapid loss of ZSM-5 additive with catalyst fines. Use of light, or low density, ZSM-5 additive will also minimize the residence time of the ZSM-5 in the riser reactor because the light catalyst will not "slip" in the riser as much as the conventional catalyst. The light ZSM-5 will be largely kept out of the regenerator, but at the price of less residence time in the riser reactor.
U.S. No. 4,336,160, incorporated by reference, reduces hydrothermal degradation of conventional FCC catalyst by staged regeneration. However, all the catalyst from the reactor still is regenerated, thus providing opportunity for hydrothermal degradation.
Although FCC processes using very active zeolite based catalysts, or mixtures of two or more zeolite catalysts are known, they have not been used much for cracking of hydrogen-deficient feeds such as resids.
Hydrogen-deficient heavy hydrocarbon feeds such as resids, syncrudes, etc., are usually cracked to more valuable products by thermal cracking, perhaps with a hydrogen donor diluent material. The hydrogen donor diluent is a material which can release hydrogen to a hydrogen-deficient oil in thermal cracking.
Resids are not routinely cracked in FCC units for several reasons, one being to much coke formation. Coke formed during catalytic cracking is usually a hydrocarbonaceous material sometimes referred to as a polymer of highly condensed, hydrogen-poor hydrocarbons. Resids make a lot of coke, and conventional FCC's can only tolerate small amounts of resid and similar materials in the feed.
Although modern zeolite cracking catalysts, e.g., using zeolites X and Y, are low coke producing catalysts, FCC's still do not tolerate much resid in the feed.
Because heavy hydrogen deficient feeds are so hard to upgrade catalytically, refiners usually resort to thermal processing as a "last resort". Visbreaking and coking are the preferred way of dealing with resids. Visbreaking reduces the viscosity of a heavy fuel fraction. Coking produces valuable liquid products, but converts a good portion of the feed to low value coke, frequently 20 to 30 wt % coke is produced.
In U.S. No. 4,035,285, a low molecular weight carbon-hydrogen contributing material and high molecular weight feedstock, e.g., a gas oil, are combined and reacted in the presence of one or more zeolite catalysts, e.g., zeolite Y with ZSM-5. The resulting cracking and carbon-hydrogen additive products are superior to those formed in the absence of the low molecular weight carbon-hydrogen contributing material. Advantages of the process include improved crackability of heavy feedstocks, increased gasoline yield and/or higher gasoline quality (including octane and volatility), and fuel oil fractions of improved yield and/or burning quality and lower levels of potentially polluting impurities such as sulfur and nitrogen. In addition, the need for high pressure hydrotreaters and hydrocrackers is reduced or eliminated.
A similar process in which full range crude oils and naphtha are catalytically cracked in the presence of such low molecular weight carbon-hydrogen contributing material and zeolites in separate risers of a multiple riser catalytic cracking unit is described in U.S. No. 3,974,062.
In spite of the many advances made, there is still a need for an FCC process which can upgrade heavy, hydrogen-deficient feeds without overwhelming the FCC regenerator with coke. It would be beneficial if mixtures of different kinds of catalyst could be used for the upgrading, with customized treatment of each kind of catalyst to maximize the potential of each catalyst.
There is also a need in the FCC process for a way to more efficiently upgrade naphtha boiling range streams in FCC reactors.
A way has now been discovered to upgrade heavy, residual refractory stocks, and to efficiently upgrade straight run and recycle naphtha fractions.